Plywood



Patented July 18, 1944 res . PLYWOQD No Drawing.

Original application August 26,

1940, Serial No. 354,25d. Divided and this application May 4, 1943,Serial No. 85,616

(Granted under the act oi March 3, 1883, as amended April 39, 1928; 37d0.. G. 75'?) '5 Claims.

This application is made under the act of March 3, 1983, as amended bythe act of April 36, 1928, and the invention herein described andclaimed, if patented, may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to us of any royalty thereon.

, This is a division of our copending application on which Patent-No.2,321,258 was granted June 8, 1943.

Our invention relates to a process for making an improved plywoodconsisting of highly compressed resin-treated surface plies that aretreated in such a way that they can be assembled and compressed witheither a treated or an untreated core in a single operation without thecorebeing appreciably compressed. This is accomplished by treating theplies that are to be compressed with a substantially unpolymerizedphenol-formaldehyde resin-forming mix that is truly soluble in water inall proportions, in such a way as to completely permeate the structure,including even that of the solid wood substance of the cell walls;slowly drying, so as to insure further diffusion of the activeresin-forming constituents into the cell-wall structure as water isremoved, at a sutflciently low temperature to avoid prematurepolymerization; followed by assembly of the treated plies, with orwithout the point of water before releasing the pressure.

The principal object of our invention is to produce a plywood in such away that part of the piles become highlycompressed and part remainvirtually uncompressed when they are subjected to a single assembly andcompression operation.

Another object of our invention is to produce a plywood, which is acombination of resintreated compressed wood and virtually uncompressedwood, in such a way that the resintreated compressed wood plies arehighly or completely compressed under a much lower pressure than thatwhich is required to produce the ordinary resln-treated compressed woodsmade according to present art.

Still another object of our invention is to produce a plywood, which isa combination of resintreated compressed wood and virtually uncompressedwood, in which the compressed 'wood plies are much less subject tomoisture adsorp tion and swelling and/or shrinking than the ordinaryresin-treated compressed woods made according to present art.

Still another object of our invention is to produce a plywood having acombination of the desirable properties of resin-treated, laminated,compressed wood and also the desirable properties of normal uncompressedwood.

Mor specifically, our objectives are:

1. To produce a light-weightplywood with a waterand weather-resistanthard glossy surface that is not subject to face checking. 1

2. To produce a plywood with a hard, glossy surface finish resultingfrom the pressing operation which is superior to the surface finishproduced by any other plywood assembly or pressing process at comparablePressures.

3. To produce a plywood. with superior mechanical properties to those ofother plywoods with the same specific gravity.

4. To produce a plywood with an extremely hard surface and resilientcore-which would be suitable for flooring.

5. To produce a plywood with a bond between compressed andvirtuallyuncompressed plies that is far stronger than could be obtainedby gluing compressed plies to virtually uncompressed plies.

6. To produce a plywood in such away that the treated plies will exhibita high degree of plasticity in the press and will conform to compoundbends such as would be required in molding airplane wings or iuselages.

According to the present art of making resintreated laminated compressedwood, the assembly and the compression of the face plies could not becarried out simultaneously without the core being highly compressed.This is due to the fact that the appreciably prepolymerized resins useddo not enter the cell-wall structure and, as a consequence, do notplasticize the cell walls. Such high pressures have to be used tocompress the treated plies, that the untreated plies are alsoappreciably compressed.

According to our process the treated plies are very much more .plasticthan those treated according to present art. This increased plasticitymakes possible the compression of the treated lies without the untreatedcores beins appreciably compressed and, as a consequence, makessimultaneous assembly and compression possible;

The simultaneous assembly and compression further makes the glue line anintegral part of the resin within the treated plies as both set to aresin simultaneously. Because of this a much better bond is obtainedthan could be' obtained by gluing resin-treated compressed wood touncompressed wood.

Differences in the manner in which we treat the veneer from thetreatment used in making the ordinary resin-treated, laminated,compressed woods account for both the differences in plasticity duringcompression and differences in moisture absorption and swelling and/orshrinking of the product. For this reason these difierences will bedescribed in detail.)

Resin-impregnated, laminated, compressed woods are made according topresent art by assembling thin sheets of veneer with appreciablyprepolymerized synthetic resin glues under such high pressures that theresin partially impregnates the coarse capillary structure of the woodwhile the compression of the plies is taking place. Under theseconditions the solid wood substance of the cell walls cannot bepenetrated with synthetic resin andeven the microscopically visiblecapillary structure cannot be uniformly treated. The process at best isone that gives a superficial filming of the fiber-cavity structure withsynthetic resin. Because of this and the fact that the synthetic resinis mechanically rather than chemically bonded to the wood, the waterresistance of these resin-impregnated, laminated, compressed woods isfar short of what it might be. The fact that the synthetic resin doesnot enter the cell-wall structure of the wood further prevents thesynthetic resin from appreciably plasticizing the wood and reducing thepressure necessary to crush the fiber structure.

Resin-treated, laminated, compressed wood made according to' ourinvention overcomes all of the aforementioned shortcomings of compressedwood made according to prior art. found that our method of treatment notonly insures the distribution of the resin-forming mix throughout thesolid wood substance of the cell walls, but also utilizes theresin-forming mix in such a way as to give a chemical bond between thepolar groups of the wood and the phenol and formaldehyde of theresin-forming mix. Because of this our treatment gives a much morecomplete and permanent protection against moisture adsorption andswelling and/or shrinking than is obtained by following the methods ofprior art.

We have also found that our treatment appreciably reduces the pressurenecessary to compress the veneer to substantially the maximumcompression because of the fact that resin-forming constituents withinthe solid wood substance of the cell walls act as plasticizers for thecompression of the fibers. We have demonstrated experimentally thatDouglas fir, spruce, and aspen veneer treated according to our method,followed by drying under conditions that will not set the resin, can becompressed to substantially the ulti mate possible compression in a hotpress under a pressure of 300 pounds per square inch.

The cell walls of dry wood contain no void structure for liquids toenter. Water, however, exhibits such a strong attractive force for woodthat his able to work its way between the structural units of the solidwood substance, pushing them apart and adding its volume to the volumeof the wood substance. This dimension change of the wood substance isthe cause of swelling We have and/or shrinking of wood. Water may carryalong certain dissolved amterials into this intimate structure of thesolid wood substance, of which the cell walls are composed, or thedissolved materials may difluse into the intimate cell-wall structurethat already contains water. We have found that solutes can enter theintimate structure of the solid wood substance in these two ways onlywhen they are in true solution and their molecular weight is small. Wehave further demonstrated that when the solute molecules have a greateraflinity for the wood than for the water, they cause the wood to swelleven more than in water alone. This is due to a greater amount ofsolution than of water alone being taken up within the intimatecell-wall structure.

We have discovered that in order to obtain appreciable and permanentreductions in the swelling and/or shrinking of wood, it is necessary touse a substantially unpolymerized resin-forming mix in which themolecules are sufliciently small to completely penetrate all of thestructure, including that within the solid wood substance of the cellwalls. We have further discovered that it is not only important that thesize of the active resin-forming molecules be small but also that theyshow greater aflinity or bonding force for wood than is shown by water,which results in swelling of the wood beyond the swelling in water. Wehave found from experiments that both aqueous solutions of phenol andformaldehyde and their substantially unpolymerized mixtures meet thesepenetration, bonding, and swelling requirements. We have alsoexperimentally demonstrated that a few commercial phenolformaldehyderesinoid mixes that are truly soluble in water in all proportions aresufliciently unpolymerized to meet these three requirements.

We believe that the polar groups of both phenol and formaldehyde attachthemselves to the polar groups of cellulose and lignin, which arenormally the seat of adsorption of water in the presence of the aqueoussolvent. When the aqueous solvent is removed on drying, most of thephenol and formaldehyde molecules remain fixed on these active groups,not only throughout the coarse capillary structure of the wood butwithin the solid wood-substance structure of the cell walls. When theresin is formed by the application of heat, the active groups of thewood also enter into the reaction to form substantially aphenol-formaldehyde-cellulose resin and a phenol-formaldehyde-ligninresin at the Junction between resin and wood throughout the structure ofthe cell walls, as well as on the cell-wall surfaces.

We have found that when the resin-forming mix is appreciablyprepolymerized it will not enter the solid wood-substance structure ofthe cell walls, and further, it will show very little tendency if any tochemically bond to even the accessible structure. For this reasonmoisture adsorption and swelling and/or shrinking are but slightlyreduced and then only over relatively short periods of time. The lack ofchemical bonding power of the appreciably prepolymerized resin is due tothe fact that the active groups of the resin-forming constituents havereacted with each other and, as a result, are no longer available tobond to the activ polar groups in the wood.

We consider the treatment of wood with a substantially unpolymerizedphenol-formaldehyde, resin-forming mix that is soluble in water in allproportions, followed by polymerization in place, to be an essentialfeature of this process as such a treating solution makes possible notonly the intimate treatment of the entire structure within the solidwood substance, which is normally accessible to water, but also thechemical bonding of the resin to the wood. This treatment is not to beconfused with the well-known treatments which merely force appreciablypolymerized phenol-formaldehyde resins into the coarse capillarystructure of dry wood with or without the aid of an organic solvent, inwhich case the resin can only be mechanically bonded to the wood over avery small fraction of the internal surface that is available to water.

We have found that veneer may be treated by either of the two generaltypes of treating methods and give a product substantially the same inall of its properties:

(1) Dry veneer or partially dry veneer can be treated in avacuum-pressure treating cylinder according to the well-known practiceused for treating wood with preservatives. The material to be treated isstacked in the treating cylinder, a high vacuum is applied, the aqueousresin-forming solution is run into the cylinder to completely immersethe material, followed, when necessary to obtain the desiredpenetration, by the application of pressure. We have experimentallydemonstrated that qs-inch Douglas fir veneer can be completely treatedin this way in 15 to 20 minutes.

(2) Undried veneer direct from the cutter knives can be treated bymerely soaking it in the treating solution at room temperature or aslightly elevated temperature (100 F.). We have found that the time ofsoaking will vary with the temperature, the amount of activeresin-forming constituents that it is desired to have diffuse into theveneer, and the moisture content, thickness, specific gravity, andspecies of the veneer. We have found that normal undried fi-inch Douglasfir veneer will require about 24 hours soaking at room temperature totake up sufilcient resinforming constituents to form 30 percent of theweight of the veneer of resin within the intimate cell-wall structure ofthe veneer. By raising the temperature to 100 F., the rate of diffusionwill be more than doubled without an appreciable deleteriousprepolymerization of the resin taking place. We have found that undrieda e-inch red gum veneer will take up about 40 percent of its weight ofresin-forming constituents by soaking for 1 hour in an aqueousresin-forming solution containing approximately 40 percent of activeresin-forming constituents when maintained at 100 F.

We have found that forming 30 to 40 percent of resin, on the basis ofthe dry weight of the wood, within the intimate cell-wall structure ofthe wood gives substantially the optimum protection against moistureadsorption and swelling. We have found that this amount of resincorresponds very closely to the amount necessary to saturate the solidwood-substance structure of the cell walls. Forming larger amounts ofresin in the wood necessitates part of the resin being formed in thefiber cavities where it is wasted. Forming smaller amounts of resinwithin the wood structure gives a proportionally smaller reduction inmoisture adsorption and swelling and does not give a satisfactory bondupon assembly of the plies without the use of additional bondingmaterial. We, hence, recommend that 30 to 40 percent of the weight ofthe dry wood of resin should be formed within the solid wood-substancestructure. I

We also recommend that in treating veneer by the pressure method thatthe resin-forming mix be diluted with water to the extent that whenabsorption is complete, the synthetic resin content of the wood will bebetween 30 and 40 percent. We have found, in the case of wood with anaverage specific gravity, that dilution of the resin-forming mix with anequal volume of water gives the desired take-up of resin-formingconstituents.

We have found that it is necessary for the resinforming constituents tobe quite uniformly distributed throughout the solid wood substance ofthe cell walls of the wood, rather than being deposited within thecoarse capillary structure, in order for the treatment to appreciablyminimize subsequent moisture adsorption and swelling. We havedemonstrated that this can be accomplished when dry veneer is treated bythe vacuum-pressure treating method by letting the veneer soak in thetreating solution for about a day after removing from the treatingcylinder, or stacking the treated veneer outside of the solution for oneto two days under non-drying conditions, that is, avoiding the settingup of a relative humidity difference between the .wood and thesurrounding air by maintaining the wood at the same temperature as thesurrounding air and avoiding the circulation of air around or throughthe wood. The pressure treatment merely carries the treating solutioninto the coarse capillary structure or the wood. Time is required forthe solution to diffuse from the coarse capillary structure of the woodinto the cell walls.

We have also found that it is necessary to dry the veneer slowly afterthe combined pressure and diffusion treatment or after the simplediffusion treatment. We have found that in order for theresin-formingconstituents to continue to diffuse into the cell walls ofthe wood as water is removed by evaporation from the coarse capillarystructure, the rate of moisture removal should be slow and steepmoisture gradients within the veneer should be avoided. We have foundthat when veneer is dried at a temperature not exceeding 120 F. andunder a relative humidity of not less than 75 percent that theresin-forming constituents continue to diffuse into the cell walls fromthe fiber cavities during the drying process,

and that premature setting of the resin is avoided. When the wood ispractically in equilibrium with the drying conditions, the relativehumidity 'ing material between faces and core.

may be dropped so as to dry the wood to about 6 percent moisturecontent.

We have discovered that veneer treated and dried according to ourinvention can be simultaneously assembled with a dry untreated core andcompressed to form'a product with highly compressed faces and avirtually uncompressed core in a single operation in a hot press heatedto 300 F. to 350 F. without the use of additional bond- The core may bea single ply or a combination of several plies that have beenpre-assembled or that are assembled with a hot-press glue at the sametime that assembly with-and compression of the face plies isaccomplished. The pressure used will depend upon the species used forthe faces and for the core as well as the degree of compression desired.We have shown that most of the softwoods (coniferous or needle-leavedwoods) can be compressed to almost their theoretical compression, afterhaving been subjected to our treatment with a resin-forming solution,under a pressure of only 300 pounds per square inch. Aspen and a fewother soft hardwoods (deciduous or broad-leaved woods) will compresswith equal case, but most of the hardwoods require greater pressure tocompletely compress the treated plies. Untreated hardwoods and softwoodswill be compressed but slightly under a pressure of 300 pounds persquare inch. IAS a result of this, we have been able to successfullymake the combination of high y compressed and virtually uncompressedplywood with several different combinations of species under a pressureof 300 pounds per square inch.

We have shown that the surface of the com pressedplies is appreciablyimproved by cooling the press platens below the boiling point of waterbefore releasing the press pressure. Evidently the hot product is stillslightly plastic and yields slightly under the stresses set up duringcooling.

When cooled in the press, this deformation of the product is impossible.We, hence, recommend that the plywood be partially cooled in the pressbefore releasing. the pressure. i

We have found yellow poplar to be a very suitable wood for the core asit has a rather high compressive strength for its specific gravity. Whentreated spruce, Douglas fir, and aspen faces were compressed onto theyellow poplar cores at pressures of 300 pounds per square inch, thefaces were compressed to about 40 percent of their original thickness,whereas the core retained 94 percent of its original thickness. Thespecific gravity of the faces was thus increased about 2.5 times,whereas the specific gravity of the core was increased about 6 percent.

The treated faces may also be made of less compressible hardwoods suchas yellow poplar. If yellow poplar is also used for the core, the facescan be only partially compressed when appreciable compression of thecore is to be avoided. In such a case it is advisable to use additionalphenolic bonding material as it is possible that insufllcientresin-forming constituents are exuded from the treated ply to form aperfect bond. Under a pressure of about 300 pounds per square inch, thetreated yellow poplar face plies are compressed to about two-thirds oftheir original thickness. Under pressures sufficient to double thespecific gravity of the poplar face plies, the untreated poplar corewould be compressed to about 80 percent of its original thickness.

We have shown that the compression of the core can be reduced and theweather resistance of the plywood as a whole increased by treating thecore with phenol-formaldehyde resin-forming constituents according tothe procedure of our invention, followed by polymerization of the resinwithin the cell-wall structure of the core by the application of heatprior to assembly of the plies. This treatment increases the compressivestrength of the core material by about 50 percent and hence willdiminish the compression occurring under a definite load. The treatmentof the core will also diminish the swelling and shrinking of the woodsubstance and, as a result, will diminish the internal stresses causedby changes in the equilibrium relative humidity.

the treating solution, the manner of insuring that the resin-forming mixdiffuses into the solid wood substance structure of the cell walls, andthe assembly and compression conditions, all of which are essential toaccomplish the objectives of our invention herein described.

Example 1.'Iake clear green spruce veneer for the plywood faces directfrom the cutter knives and immerse in an aqueous solution consisting ofequal parts by weight of phenol, formaldehyde (aqueous 40 percent), andwater together with a suitable catalyst such as hexamethylene tetramine(about 2 percent) at about 1'. Let the veneer soak in the solution for aperiod of time necessary for the wood to take up by diffusion an amountof the resin-forming mix necessary to form 30 to 40 percent of theweight of the dry wood of resin within the wood structure. Remove thewood from the treating bath and slowly dry at a temperature not toexceed F. and a relative humidity of at least 75 percent. When theveneer is practically in equilibrium with these conditions, the relativehumidity should be dropped to about 30 percent so as to bring the veneerto a moisture content of about 6 percent. Take yellow poplar dried to amoisture content of 6 percent or less for the core. Assemble this withthe treated spruce faces without the use of additional bonding materialin a parallel or cross-banded manner in a hot press at a temperature of310 F. and a pressure of 300 pounds per square inch for a period of 15minutes. Cool the press platens below 212 F. and withdraw the finishedproduct with compressed resin-treated faces and only a slightlycompressed core.

Example 2.Take clear dry or partially dry spruce veneer and stack it onedge in a vacuumpressure treating cylinder with some form of spacersbetween the sheets of veneer to hold them slightly apart. Apply a highvacuum on the cylinder and then run in the treating solution identicalto that given in Example 1 so as to completely immerse the veneer. Afterabout 20 minutes remove the veneer from the treating cylinder and stackit and permit it to stand for one to two days under nondryingconditions, that is, at room temperature in a room freefrom aircirculation. Dry, assemble, and compress the veneer according to theprocedure given in Example 1.

It will be noted that the foregoing examples are directed toward thetreatment with a resin-forming mix consisting of equal parts of phenol,formaldehyde, and water containing 2 percent hexamethylene tetramine ascatalyst. We may vary the proportion of phenol, formaldehyde, and waterin the treating mix over an appreciable range, substitute aqueousammonia or other alkalies such as sodium hydroxide for the hexamethylenetetramine catalyst, and appreciably change the concentration of thecatalyst and attain substantially the same results as are obtained inExample 1. We may also substitute cresol for phenol or.

we may use an aqueous solution of a commercialphenol-formaldehyde-catalyst mix that is substantially unpolymerized andcompletely soluble in water in all proportions in place of the treatingsolution described in Example 1.

It will be further noted that'the foregoing examples are directed towardthe use of spruce for the faces and yellow poplar for the core. We haveshown that any species of veneer which is readily compressed in a hotpress aftrreceiving our treatment may be substituted for the sprucefaces and give a product with highly compressed faces under the pressingconditions given. We

the pressing conditions given, we have found it advisable to useadditional bonding material between the plies as insufilcient bondingmaterial exudes from the treated faces because of the reducedcompression. We have also shown that the pressing pressure may beappreciably increased to more completely compress the less compressibletreated faces if a partial compression of the core is not considereddetrimental for the purpose to which the material is to be put. The coremay further consist of a single ply or multiple plies that have beenpre-assembled or that are assembled with the use of a hot-press glue atthe same time that assembly with and compression of the face plies isaccomplished.

It will also be noted that the foregoing examples are directed towardsthe use of treated faces with an untreated core. We have shown that amore weather-resistant product and a less compressed core can beobtained if the core is also treated and dried in the same manner as theface plies, followed by heating to about 200 F. for several hours priorto assembly so as to prepolymerize the resin.

Having thus described our invention, we claim:

1. A wood product consisting of dense, compressed, permanentlywater-resisting phenolformaldehyde resin-treated face-plies, having hardweatherand chemical-resistant, smooth finished surfaces and adistribution of resin throughout the cell-wall structure, the resin being chemically bonded to the polar groups of the wood constituents ofthe face plies, which face plies are integral with an untreated,substantially uncompressed core, the resin bond being chemicallycontinuous with the resin throughout the face plies.

2. A wood product consisting of dense, compressed, permanentlywater-resistant phenol= formaldehyde resin-treated face plies, havinghard weatherand chemical-resistant, smooth finished surfaces and adistribution of resin throughout the fine cell-wall structure, the resinbeing chemically bonded to the polar groups of the wood constituents,which face plies are integral with a similarly treated, uncompressedcore, the resin bond being chemically continuous with the resinthroughout the face plies.

3. A wood product consisting of a dense, compressed, permanentlywater-resistant phenolformaldehyde resin-treated face-ply, having a hardweatherand chemical-resistant, smooth finished surface and a thoroughdistribution of resin throughout the fine cell-wall structure, the resinbeing chemically bonded to the polar groups of the wood constituents,which face ply is integral with an untreated, substantially uncompressedcore, and with an uncompressed second face ply treated similarly to thefirst, the compressed face ply-uncompressed core resin bond beingchemically continuous with the resin throughout the compressed face ply.

4. A 'wood product consisting of a dense, compressed, permanentlywater-resistant phenolformaldehyde resin-treated face-ply, having hardweatherand chemical-resistant, smooth finished surface, and adistribution of resin throughout the cell-wall structure, the resinbeing chemically bonded to the polar groups of the wood constituents,which face ply is integral with a similarly treated, virtuallyuncompressed core and second face ply, the compressed face ply-coreresin bond being chemically continuous with the resin throughout thecompressed ply.

5. A laminated wood flooring consisting of a dense, compressedphenol-formaldehyde resintreated upper face ply, having a hard waterandchemical-resistant. smooth finished surface and a distribution of resinthroughout the cell-wall structure, the resin being chemically bonded tothe polar groups of the wood constituents of the face ply, which faceply is integral with an untreated, virtually uncompressed core and withan under surface ply treated similarly to the upper face ply, virtuallyuncompressed, the compressed upper face ply-uncompressed core resin bondbeing chemically continuous with the resin throughout the compressedupper face ply.

ALFRED J. STAMM. RAYMOND M. SEORG.

